The invention relates to a thyristor with amplifying gate including a silicon semiconductor body and n.sup.+ conductive emitters at the cathode side. The invention further relates to a method for making such a thyristor.
Thyristors with internal firing amplification, usually called thyristors with amplifying gate, are used instead of the much more complicated firing pulse generators in order to effect firing of power thyristors with as high a control current as possible.
In thyristors with amplifying gate, an auxiliary thyristor is integrated into the semiconductor body. This auxiliary thyristor is initially fired with a low control current. The load current of the auxiliary thyristor, which is obtained from the load circuit and not from the control circuit, then acts as a high control current for firing the main thyristor. Within a short period of time, the latter takes over the load current from the auxiliary thyristor and protects the auxiliary thyristor against destruction due to overload. This transfer of control increases the size of the primarily fired cathode surface of the main thyristor which can be given relatively large dimensions. This reduces the turn-on power loss of the thyristor and the permissible current rise characteristic di/dt.sub.crit is increased compared to that of the conventional thyristor without a structure incorporating internal firing amplification.
When designing a thyristor with an amplifying gate, suitable measures must be taken to assure that the auxiliary thyristor will fire before the main thyristor in any case. If this does not occur, the main thyristor will receive a weak current, will fire irregularly on small surface regions and may be destroyed by thermal overloads in the initially fired channels.
With correct dimensioning of the auxiliary thyristor it is assured that its permissible load current always provides a sufficient transfer of control to uniformly fire the main thyristor. The edge region of the main emitter facing the auxiliary emitter can therefore be given relatively large dimensions without having to impress a high external control current so as to uniformly fire the entire edge region of the main cathode.
Various solutions for this problem have been published in literature. The criteria for dimensioning a thyristor with amplifying gate were given by R. A. Kokosa and E. D. Wolley, in IEEE, Intern. Electr. Dev. Meeting, 1974, pages 431-434. A typical thyristor with amplifying gate is shown in FIG. 1. In this component, the semiconductor layers 6, 8, 9 of alternating conductivity type (p,n,p.sup.+) and the metallic anode contacts 10 support the control electrode 4, the metallic contact 7 of the auxiliary emitter 1 as well as the metallic cathode contact 5 of the main emitter 2. The main emitter 2 is provided with several short-circuiting holes 11. Decisive for the operation of the amplifying gate are the resistances of the trigger zone 6 between control electrode 4 and auxiliary emitter 1, R.sub.gp, underneath the auxiliary emitter 1, R.sub.p, between the auxiliary emitter 1 and the main emitter 2, R.sub.pm, and underneath the edge of the main emitter 2 and the first short-circuiting hole 11, R.sub.m. These resistances are shown in dashed lines in FIG. 1. An equivalent circuit diagram for such a thyristor is shown in FIG. 2 according to the data given by Victor A. K. Temple and Armand P. Ferro, in IEEE Trans. Electr. Dev., Vol. ed-23, No. 8, August 1976, page 893, in which the control electrode is marked G and the cathode is marked K.
In the state before turn-on (I.sub.A .apprxeq.0; I.sub.G =I.sub.K) the control current I.sub.G flows laterally through the resistors of the p-conductive trigger zone 6. It produces a voltage drop V.sub.p across resistor R.sub.p and a voltage drop V.sub.m across the lateral base resistor R.sub.m. When V.sub.p or V.sub.m reach a value of about 0.7 V (at room temperature) and under usual doping conditions, that thyristor stage will fire at which 0.7 V appears first. Consequently, an amplifying gate is designed correctly, if the following always applies: EQU V.sub.p &gt;V.sub.m.
The prior art solutions are only based on the fact that this is assured by the appropriate setting of R.sub.p and R.sub.m, i.e. the geometric dimensions are selected so that EQU R.sub.p &gt;R.sub.m
applies. R.sub.p can be increased by increasing the lateral expanse of the auxiliary emitter 1 or by diffusing the n.sup.+ zone of the auxiliary emitter 1 deeper and thus decreasing the conductivity of the p zone below this n.sup.+ zone 1. A further known measure aiming at reducing R.sub.m resides in the fact that the first short-circuiting holes 11 in the n.sup.+ region of the main emitter 2 are placed very close to the edge of the main emitter 2 facing the auxiliary emitter 1 and the short-circuiting holes 11 are given a relatively large diameter.
All known measures for obtaining a greater voltage drop underneath the auxiliary emitter 1 than under the main emitter 2 have drawbacks with respect to the dynamic properties of the thyristors, the critical voltage rise rate du/dt and the critical current rise rate di/dt as well as the recovery time t.sub.q.
If, for example, the lateral expanse of the auxiliary emitter 1 is increased and the spacing between the main emitter edge and the first short-circuiting hole 11 in the main emitter 1 remains the same, the resistance R.sub.p will increase compared to R.sub.m. Although this has the result that the auxiliary thyristor always fires before the main thyristor, there do exist the drawbacks which will be explained in connection with FIG. 3. The reference numerals of FIG. 3 correspond to those of FIG. 1. When a voltage is applied across the center pn junction between layers 6 and 8, a homogeneous shifting current of holes flows vertically out of the developing space charge zone toward the n.sup.+ emitter. The current density of this hole current is constant over the entire surface of the pn junction. Its absolute magnitude is determined by the voltage rise characteristic du/dt.
It corresponds to the state of the art to select the short-circuiting density in the main emitter 2 in such a manner that with the desired voltage rise characteristic all holes flowing under the region of the main emitter 2 are conducted away via the short-circuiting points 11 without producing an injection of electrons from the main emitter 2.
In the area of the thyristor which does not lie underneath the shorted main emitter 2, the hole current flows out of the developing space charge zone at the same vertical current density as underneath the main emitter. In contradistinction of the conditions underneath the main emitter 2, in this region the holes cannot flow out through shorting channels since the auxiliary emitter 1 is not connected to the external voltage source. The entire hole current from this area must flow away laterally through the p trigger or control base zone 6 to the first shorting holes 11 of the main emitter 2. The lateral current density then grows continuously from control contact 4 toward the main emitter 2, which is shown in FIG. 3 by an increase in the number of arrows symbolizing the current density. The current which is summed up to the outer edge of the auxiliary emitter is marked I.sub.p, the current entering underneath the main emitter is marked I.sub.m.
A large lateral expanse of the axuiliary emitter 1 always results in a large lateral hole current. Thus, with a higher voltage rise characteristic du/dt, or load, the fact that the recovery time t.sub.q is not reached will quickly result in exceeding the critical voltage of 0.7 V due to the voltage drops I.sub.p .multidot.R.sub.p across R.sub.p or I.sub.m .multidot.R.sub.m across R.sub.m, respectively, i.e. as a result of the lateral hole current, the thyristor switches too early although it did not receive a control current.
Similar drawbacks, particularly in the blocking behavior of the thyristor, result if the increase in R.sub.p is set by way of a greater depth of the auxiliary emitter 1, i.e. if, for example, the n.sup.+ zone of the auxiliary emitter 1 is diffused deeper into the p zone 6 and thus the conductivity of the p zone 6 underneath this n.sup.+ zone is reduced. Although this thyristor will always fire first in the auxiliary emitter 1, it has no better du/dt and t.sub.q values, if the length of the inner edge of the main emitter (which is set by the required di/dt) is the same, than the thyrsitor with a laterally expanded auxiliary emitter. The product I.sub.m .multidot.R.sub.m is not changed by this measure, i.e. the thyristor fires at the same values of the lateral hole current I.sub.m as the previously discussed thyristor with laterally expanded auxiliary emitter.
Instead of increasing the resistance R.sub.p in the region below the auxiliary emitter 1, measures have also become known to reduce the resistance R.sub.m in the area of the main emitter 2. This is accomplished in that the first shorting holes 11 in the n.sup.+ region of the main thyristor 2 are placed very close to the edge facing the auxiliary emitter and the shorting holes 11 are given a large area. Such measures which are based on increased shorting of the main emitter 2, have the drawback, however, that the propagation speed of the turn-on process is reduced since only part of the edge of the main emitter 2 carries current at the beginning of the turn-on process, which results in greater turn-on losses.
Finally, there are diffused ohmic connections in the form of strip-shaped current paths or bars between the auxiliary and main emitter as described, for example in German Offenlegungsschrift [Application published without examination] No. 2,329,872. This is an ohmic current path disposed at the surface of the semiconductor body and not interrupted by a pn junction. With n.sup.+ emitters, this bar can thus only be designed as an n conductive region. This bar has the effect of a fixed resistor which connects the auxiliary and main emitters together. FIG. 4 is a top view of an embodiment with a bar 13. The reference numerals correspond to those of FIG. 1. The pn.sup.+ junction disposed underneath the auxiliary emitter contact 7 between the trigger zone 6 and the auxiliary emitter 1 has the reference numeral 12. The load current I.sub.L1 flows through the bar 13, the load current I.sub.L2 flows through the p conductive region 6. The load currents I.sub.L1 and I.sub.L2 are each components of the total load current I.sub.L.
The operation of the n bar will be explained in detail with the aid of an equivalent circuit diagram as shown in FIG. 5. The resistors R.sub.p, R.sub.m and R.sub.s are assumed to be selected so that it is assured that the auxiliary thyristor is always fired before the main thyristor. The load current I.sub.L of the auxiliary thyristor flowing through the anode A, as shown by the arrow, initiates the firing of the main thyristor.
R.sub.s indicates the resistance of the n bar 13; R.sub.pm and R.sub.m have the significance described above. The load current I.sub.L flowing in the auxiliary thyristor is thus divided into the two components I.sub.L1 and I.sub.L2 corresponding to the resistance relationships R.sub.s and R.sub.pm +R.sub.m in the parallel circuit. The component I.sub.L1 flows out, without having the effect of a control current for the main thyristor, via the bar 13 from the auxiliary thyristor to the main thyristor. Only the component I.sub.L2 causes the main thyristor to fire.
In order to realize good du/dt and t.sub.q properties for the thyristor, R.sub.s must be selected to be small compared to R.sub.n +R.sub.pm so that a sufficiently large portion of the capacitive hole current developing in the area not underneath the main emitter 2 can flow off via R.sub.s without producing an injection at the main emitter.
A low R.sub.s value leads to the following general drawbacks: when firing with control current, the auxiliary thyristor must carry a correspondingly higher load current before the main thyristor fires. The firing ineffective component I.sub.L1 is enlarged and produces undesirable heating of the auxiliary thyristor. At higher frequencies this may lead to an overload and thermal destruction of the auxiliary system.
If this general drawback is accepted, there remain in addition further worsenings regarding the di/dt or du/dt behavior of the thyristor depending on its geometric design. In order to attain a low R.sub.s, either one or a few broad bars can be applied, or a plurality of narrow bars. One broad bar would reduce the critical du/dt value since the bar acts as an n.sup.+ emitter and is not short-circuited. With many narrow bars, however, the main thyristor would fire, at the inner edge opposite the auxiliary emitter, only between the bars. This reduces the critical di/dt value. The bars thus have a similar effect at the inner edge of the main emitter as the shorting holes.